1. Field of the Invention
The present invention relates to a method for generating backscattering intensity on the basis of a lower layer structure in a charged particle beam exposure, and to a method for fabricating a semiconductor device that utilizes this method. More particularly, the present invention provides a more accurate method of determining the backscattering intensity by considering the influence of charged particles on the surrounding area in the case of a lower layer structure with a plurality of layers, and provides a method for fabricating a semiconductor device that enables more precise proximity effect correction.
2. Description of the Related Art
In recent years, the increased integration of semiconductor devices has been accompanied by increased intricacy in the pattern size required, hence the resolving power of conventional exposure methods using light is inadequate, and the formation of fine patterns is becoming problematic. Therefore, exposure methods that employ a charged particle beam and, more particularly, an electron beam are now being used. Electron beam exposure methods include point beam exposure which has a high resolution but a low throughput; variable shaped beam exposure, which raises the throughput by exposing the pattern in small rectangular units; partial batch exposure methods (block exposure methods), which subject an emerging patterns that is repeated in a chip to a batch transfer by using a stencil mask; and projection-type exposure methods, which create a mask for the whole pattern in the same manner as in optical exposure and then perform a large area batch transfer, and so forth. With the partial batch exposure methods and projection-type exposure methods, the electron beam shot number can be reduced, and hence the throughput can be raised. However, a drop in the exposure accuracy is readily brought about because it is not possible to change the exposure amount in accordance with the exposure pattern.
One problem that is common to such charged particle beam exposure methods (simply ‘electron beam exposure methods’ hereinbelow) is that the dimensions of the resist fluctuate due to the proximity effect. The electrons that pass through the resist are scattered by the substance constituting the substrate and return inside the resist, meaning that the resist is re-exposed. The amount of electrons returning to the resist is proportionate to the sparseness or denseness of the pattern, and hence the margin of fluctuation of the resist dimensions caused by the proximity effect also varies in accordance with the pattern layout.
As a method for correcting the proximity effect, a method has been proposed that estimates the influence of the electrons returning from the substrate in each area on the basis of an energy distribution function (EID: Exposure Intensity Distribution) in which electrons entering from one point are supplied to the resist, optimizes the exposure amount of this area accordingly, or changes the pattern dimensions, and so forth. This method appears in Japanese Laid Open Patent Publication No. 2001-52999 (published on Feb. 23, 2001) and the corresponding American patent (U.S. Pat. No. 6,610,989 issued on Aug. 26, 2003), as well as in Japanese Laid Open Patent Publication No. 2002-313693 (published on Oct. 25, 2002) and the corresponding American Laid Open Patent No. 2002-0177056 (published on Nov. 28, 2002), which will be described later, for example.
The EID function is generally known and expressed by the sum of two Gaussian distributions, i.e. by the following equation, when the substrate is constituted by one type of substance.
                              f          ⁡                      (                          x              ,              y                        )                          =                              1                          π              ⁡                              (                                  1                  +                  η                                )                                              ⁢                      {                                                            1                                      β                    f                    2                                                  ⁢                                                                  ⁢                                  exp                  ⁡                                      (                                          -                                                                                                    x                            2                                                    +                                                      y                            2                                                                                                    β                          f                          2                                                                                      )                                                              +                                                η                                      β                    b                    2                                                  ⁢                                                                  ⁢                                  exp                  ⁡                                      (                                          -                                                                                                    x                            2                                                    +                                                      y                            2                                                                                                    β                          b                          2                                                                                      )                                                                        }                                              (        1        )            
Here, βf is the forward scatter length, βb is the backward scatter length, and η is the forward and backward scatter ratio. The first term represents the energy supplied to the resist by the incident electrons, and the second term represents the energy supplied to the resist by the electrons reflected by the substrate, these terms being respectively known as the forward scatter term and backward scatter term.
When electron beam exposure is performed in a semiconductor integrated circuit (LSI) fabrication process, a structure consisting of wiring and contact plugs and so forth is already created in layers below the resist, and the substrate is constituted by a plurality of substances. Because, if the substances constituting the lower layers are different, the parameters of the backscatter term are different, it is not possible to estimate the influence on the exposure intensity distribution by means of a simple Gaussian distribution function such as that shown in Equation (1) above. In order to resolve this problem, a procedure that carries out proximity effect correction by considering the structure one layer below the resist has been proposed (J. Vac. Sci. Technol. B, Vol. 10, No. 6, pages 3072 to 3076 (1992) described later, for example). According to this procedure, in a case where the structure one layer below the resist is constituted by tungsten (W) contact plugs and a silicon oxide film (SiO2) that is embedded between the W plugs, for example, when the backscattering intensity is determined by means of the area density method, the influence of the backscattered electrons that return from an area in which the exposure pattern density is α and the W density of the lower layer is αw is determined by the following equation.α(αwηw+(1−αw)ηSiO2)  (2)where ηw and ηSiO2 are forward and backward scatter ratios determined in a W or SiO2 state.
However, the substances that constitute the wiring, contact holes and so forth are normally equal to or less than 1 um (where ‘μ’ has been expediently written as ‘u’) and thin, and therefore the electrons reach even lower layers. When electrons that have passed through a single layer return to the resist, because the electrons are affected by the wiring and contact holes in the process, the backscattering intensity cannot be determined simply by means of Equation (2) above. For example, when a layer that is one layer below the exposed Al layer to be patterned is constituted by W contact holes and SiO2, which is embedded between these contact holes, the W readily scatters the electrons and hence few electrons escape to lower layers. On the other hand, electrons entering the SiO2 enter relatively deeply. However, electrons entering the SiO2 deeply are scattered by the W contact holes in the process of returning to the resist. Therefore, the quantity of electrons that return after being backscattered is reduced in comparison with a case where the whole of the lower layer consists of SiO2. In addition, because the LSI wiring structure also consists of, not two layers, but any number of stacked layers, the electrons that escape the layer one layer below are also affected by layers two or three layers below.
Therefore, a determination of the backscattering intensity by considering the plural layer structure of the layers below the resist layer has been proposed (‘Emerging Lithographic Technologies VII, Roxann L, Engelstd, Editor, Proceedings of SPIE Vol. 5037 (2003)’ (described later, for example). In the case of the procedure mentioned in this prior publication, areas are classified in accordance with the lower layer structure in order to consider the plural layer structure of the lower layers. For example, such areas include areas where W is not present in a lower layer, areas where W is present only one layer below the resist, areas where W is present only two layers below, and areas where W is present both one and two layers below, and so forth. Thereafter, backscattering calculations using the area density method are performed by using forward and backward scatter ratios and backward scatter lengths that differ for each area.
Further, the addition of marginal exposure so that the backscattering intensity is made uniform by considering the plural wiring layer structure that exists in the layers below the resist layer has also been proposed (Japanese Laid Open Patent Publication No. 2950280 (published on Sep. 20, 1999) and the corresponding American U.S. Pat. No. 6,243,487 B1 (published on Jun. 5, 2001), for example). In this prior publicatio, the backscattering intensity is made uniform irrespective of the lower-layer wiring layer structure by generating marginal exposure in accordance with the respective pattern densities of the plurality of wiring layers of the lower layers.